Strange that WSDL->Java is producing that interface given that the source WSDL is actually using document literal with one part for the message. In other words, the source WSDL has only one "parameter" so to speak. My recommendation would be to use the generated Java interface as a starting point and patch up any multi-parameter operations.
Out of curiosity, did you try the "Create wrapper for message parts" option? Doesn't seem like it should make a difference in this case, since there's only a single part.